In a conventional spinning unit designated generally by a reference numeral 1 (FIG. 1), sliver 3 or a bundle of fibers is supplied through the sliver feeding inlet 2 and transferred by the feed roller 4 operating cooperatively with its presser 5 toward the combing region, where it is subjected to combing action by the combing roller 6 and, simultaneously, foreign matter or impurities 7 contained therein, such as fragments of leaves or seeds, are removed therefrom. Such foreign matter is discharged through the trash outlet 0. The fibers which have been thus opened or separated into individual fibers by the combing roller 6 are then carried by an air stream 12 into a cavity, formed in the rotor 9, which defines the spinning chamber 10 of the rotor. The flow of air 12 is developed in the fiber passage 11 due to the vacuum created upon high speed rotation of the spinning chamber 10. Such high speed rotation also creates a rotary airstream within the chamber 10 to which the introduced fibers are subjected. The fibers are thus carried outwardly into contact with the interior peripheral sidewall surface 9a of the chamber, and forced by centrifugal action to slide downwardly on the peripheral surface 9a to the fiber-collecting groove 13 which is formed at the maximum-diameter region within the spinning chamber 10. They are deposited in the form of a ribbon within the groove 13 and are subsequently withdrawn as a strand of twisted, elongated yarn through the yarn exit hole 14.
In the above-described process of open-end rotor spinning, there are two known methods of creating vacuum in the spinning chamber 10. One is a so-called forced-exhaust method, according to which air in the spinning chamber 10 is drawn off in a positive manner using a suction device (not shown) connected to an exhaust port 16 formed in the rotor casing 15 which encloses the spinning rotor 9. The other is a self-exhaust method in which a plurality of exhaust vents 17 are formed at the bottom 9a of the rotor, each extending in an outward radial direction of the rotor to provide air vent communication between the spinning chamber 10 and the exterior or the spinning unit 1. Air within the spinning chamber 10 is automatically drawn off through these exhaust vents 17 by the centrifugal force developed in response to the high speed rotation of the rotor 9, thereby creating vacuum in the chamber 10. In either of these methods, it is necessary that the fibers which are carried into the spinning chamber 10 from the fiber passage must reach the interior peripheral surface 9a of the rotor as rapidly as they possibly can. In the conventional open-end spinning apparatus, however, some of the fibers introduced into the spinning chamber 10 with the air stream 12 are not picked up immediately by the whirling air within the spinning chamber 10 which is intended to carry all of the fibers onto the interior peripheral surface 9a. Instead, some of such fibers are deposited in the fiber-collecting groove 13 in a bent or broken form, while the other fibers remain floating in a free state within the spinning chamber 10. As a result, bent fibers will inevitably be included in the spun yarn, thereby reducing the strength of the yarn product, and the fibers which are floating in the chamber 10 are caught by the spun yarn as it is being withdrawn toward the yarn exit 14, thereby also adversely affecting the quality of the resulting yarn.
In open-end spinning, particularly when the self-exhaust method is used, the static pressure within the spinning chamber 10 is lower in the vicinity of the axis of rotation of the rotor 9 and is higher near the interior peripheral surface 9a thereof, as indicated by the diagram of FIG. 2. Accordingly, the fibers introduced into the spinning chamber 10 from the fiber passage 11 have a tendency to be drawn toward the exhaust vents 17 around which the static pressure is relatively low, and it is more difficult for the fibers to be forced by centrifugal action against the interior peripheral surface 9a of the rotor 9, as intended.
It is known that if the length of each such exhaust vent 17 is made shorter, the rate at which air in the spinning chamber 10 is discharged through the vents 17, i.e. the volume of air being discharged therethrough, will be reduced accordingly. (For this phenomenon, refer to page 408 of "Collection of Textile Date", published by the Japanese spinners' Association, Oct. 1, 1971). For this reason, conventional spinning rotors 8 are designed having their exhaust vent openings on the rotor bottom 9a located inwardly or closer to the rotational axis of the rotor 9 with a view to maintaining the desired flowrate of air to be discharged through the exhaust vents 17.
Regarding variations in the speed of the whirling stream of air within the spinning chamber 10, it has been believed heretofore that, because of its viscosity, its speed is increased progressively, but non-linearly toward the interior peripheral surface 9a, as shown by the phantom curved line in the diagram of FIG. 3. In FIG. 3, the straight line shows the linear increase in speed of the rotary air stream which accompanies an increase in the speed of the rotor 9.
However, results of tests using a Pitot static tube have revealed that, in the spinning chamber 10, although one rotating air stream having considerably high speed is created along the boundary region in close proximity to the fast-moving internal peripheral surface 9a, at other locations within the chamber there exists only the slower rotating air stream that is formed by the flow of air into the exhaust vents 17. These results are represented by the solid line in the diagram of FIG. 3. Where the openings of the exhaust vents 17 on the bottom 9b are located closer to the rotational center of the rotor 9 for the reason previously mentioned, the resulting rotary stream will be produced in the vicinity of the center of the rotor 9. Therefore, between the rotary air stream produced by the flow of air into the exhaust vents 17 and the high speed rotary air stream created in close proximity to the interior peripheral surface 9a, there also exists an accompanying, rather large region within which the speed of the rotating air stream is even lower, as indicated by the trough portion of the solid line in the diagram of FIG. 3.
Thus, using a spinning rotor 9 having a self-exhaust system, the fibers carried from the fiber passage 11 into the spinning chamber 10 are caused to reduce their speed as they move to the region within the spinning chamber 10 which corresponds to the above-mentioned trough portion of the graph, in which region the speed of the rotary stream is reduced. Consequently, the numbers of those fibers which are collected in the fiber-collecting groove 13 in a bent form, and which remain floating within the spinning chamber without reaching the fiber-collecting groove, are increased. As a result, the fibers which constitute the spun yarn will include such bent fibers, thus reducing the strength of the yarn, or will catch the floating fibers while being withdrawn from the fiber-collecting groove 13, thereby degrading the quality of the resulting yarn.